Direction finding is one of the oldest and most widely used form of navigational aid. The direction of a transmitter with respect to an aircraft may be determined by comparing the arrival time of its transmission at two or more known points. In the simplest practical system, these two points are the vertical arms of a loop antenna connected to a receiver. As the loop is rotated, the received signals cancel each other when the plane of the loop is at right angles to a radial extended to the transmitter station. The major disadvantage of loop antenna direction finding systems is the 180-degree ambiguity problem, wherein the loop antenna can determine the bearing (i.e., the radial along which the transmitter is located), but cannot determine the direction (i.e., whether the transmitter is in front of or behind the, receiver).
An Automatic Direction Finder is a device used as an aid to determine the geographical location of the aircraft relative to a ground-based omnidirectional radio transmitter. However, in order to resolve the 180-degree ambiguity which results from the use of a primary loop antenna for determining direction, additional antennas must be used. One such additional or supplemental antenna commonly used is a "sense" antenna. Additional sense antennas can be conveniently attached to the metal surfaces of conventional aircraft to assist in resolving the ambiguity problem. However, in newer types of aircraft having composite skin structures, the attachment thereto of radio antennas is more troublesome. In particular, conventional antennas cannot be attached to the exterior surface of a composite aircraft without exacting a weight penalty, a wind drag penalty, or other disadvantage. U.S. Pat. No. 4,507,341 describes the problems and a solution in attaching the base of an aerial type of radio antenna to a composite fiber-resin type of skin and achieve a low resistance therebetween.
It is therefore an objective of the present invention to efficiently and economically provide a sense antenna to be used on composite aircraft which conforms to the shape of the aircraft skin. It is also an object of the present invention to utilize a metallized portion of the composite aircraft as an antenna element for an Automatic Direction Finder navigation receiver.